Metallurgy-grade silicon is produced by mixing carbon and quartzite and reducing the mixture in an arc furnace. The metallurgy-grade silicon is reacted with HCl to synthesize trichlorosilane, the trichlorosilane is distilled and refined, and then reduced with hydrogen at a high temperature to produce semiconductor-grade silicon. Solar-grade silicon has been mainly made of off-spec grade material supplied in producing the semiconductor-grade silicon.
While the method for producing the semiconductor-grade silicon as described above can form silicon with extremely high purity, the method requires high cost for the reasons that the rate of conversion into silicon is low and a large amount of hydrogen is required to achieve equilibrium advantageous for silicon, a large amount of unreacted gas needs to be recirculated for use because the conversion rate is still low, various types of halogenated silane produced in the unreacted gas require separation by distillation, a large amount of silicon tetrachloride which cannot be reduced ultimately with hydrogen is produced, and the like.
A solar cell has received attention as a predominant means for solving environmental issues such as carbon dioxide in recent years and has been rapidly growing in demand. The solar cell, however, is expensive at present and thus provides electric power at prices which are several times higher than the charge for electricity of commercial electric power. Since the demand for the solar cell is growing in response to the environmental issues and an increasing demand for energy, only the conventional irregular items of semiconductor silicon cannot provide a sufficient amount of a raw material of the solar cells, so that there is a need to supply a large amount of low-cost material of the solar cells.
In order to solve the above problems, a method for synthesizing highly pure carbon and highly pure silica, and then reducing it in a reduction furnace with a highly pure furnace material to synthesize highly pure silicon has already been proposed. The method, however, causes problems such as difficulty in scale-up, a low yield, and difficulty in reducing cost. Another method for reducing silicon tetrachloride with aluminum has been also proposed (see the following Non-patent Document 1, Patent Document 1, and Patent Document 2). However, in this method phosphorus in aluminum remains in silicon and makes it difficult to achieve a desired purity.
Other proposals have been made such as a method of reduction of silicon tetrachloride with zinc (see the following Non-patent Document 2) and reduction of trichlorosilane in a fluidized bed (see the following Non-patent Document 3), but none of them have been put in practical use.
A method for producing silicon by electrolysis of silica has been also studied.
Non-Patent Document 1: Reduction of silicon tetrachloride with aluminum by YOSHIZAWA Shiro, HASHINO Tomoyasu, and SAKAGUCHI Shin, Journal of the Chemical Society of Japan, Kogyo kagaku zasshi 64(8) 1347-50 (1961)
Patent Document 1: JP-A-2-64006
Patent Document 2: JP-A-59-182221
Non-patent Document 2: Evaluation of selected chemical processes for production of low-cost silicon, J. M. Blocher, et al. Jet propulsion laboratory final report (1981)
Non-patent Document 3: Solar-electric power generation system commercialization technology development: Low-cost silicon experiment refinement study, Summary of report on commissioned project of New energy and industrial technology development organization (1980-1987), by Shin-Etsu Chemical Co., Ltd. (1988)